Apparatus for molding composition resistors



April 25, 1967 3,315,309

APPARATUS FOR MOLDING COMPOSITION RESISTORS M. J. BRAUN 2 Sheets-Sheet 1Original Filed Nov. 22, 1965 FIG. 7

l/Vl/E/VTOR MAR/NUS J B/PA UN APPARATUS FOR MOLDING COMPOSITIONRESISTORS 2. Sheets-Sheet 2 M. J. BRAUN April 25, 1967 Original FiledNov. 22, 1963 FIG. 8

FIG. II

K m N R J WW IS A u W y m a M United States Patent 1 Claim. (Cl. 18I6.5)

. This is a division of application Ser. No. 325,643, filed Nov. 22,1963.

This invention relates to an apparatus for molding composition resistorsand the like that are provided with parallelly arranged axiallyextending external connecting leads.

It is now common practice to mount resistors and other circuitcomponents upon board panels, particularly printed circuit boards, whichboards carry printed, electrolytically deposited, etched or otherwiseprepared interconnecting conductors, the components being connected inplace on the board by inserting the leads from the component throughprepared holes in the circuit board and completing the electricalconnection of the component into the circuit by suitable means such asdipsoldering. By this procedure, usually a large number of circuitcomponents may be soldered in place with a single dip-solderingoperation.

Since the time before such circuit assembly methods were developed,circuit components have conventionally been provided with coaxialexternal connecting leads. For example, the usual circuit component hasbeen cylindrical in shape with a lead protruding from each end.

To mount the conventional circuit components upon a circuit board it hasbeen necessary to bend the leads to bring them into position to threadthrough the proper holes in the circuit board. The result may be thatundesirably long leads are required or that the circuit component whenin place takes up too much space. In any event, the substitution ofparallel leads extending axially from one end of the component makes thecomponent easier to handle during assembly upon a circuit board andenables the component and associated leads to take up less space in thefinished device.

In making a molded composition resistor in accordance with the apparatusof the invention, I place powdered resistive material in a U-shapedconfiguration within a partially molded body of granular insulatingmaterial in a mold according to a particular procedure to be describedbelow, place parallel leads in position in con tact with the resistivematerial, and further mold and set the assembled parts into a unit ofthe desired form. The process and assembly procedure involve the use ofpunches and dies of special form to effect the desired results.

Other features, objects and advantages will appear from the followingmore detailed description of an illustrative embodiment of theinvention, which will now be given in conjunction with the accompanyingdrawings.

In the drawings,

FIG. 1 is a perspective view, partly broken away, of an illustrativeresistor made in accordance with the present invention;

FIG. 2 is a vertical section through a mold and punch parts, showing themethod of forming a cavity to receive the powdered insulating materialin the initial molding operation;

FIG. 3 is a cross-sectional view of a shell pin appearing in axialsection in FIG. 2, taken along the line 33 in that figure;

FIG. 4 is a bottom view of a top punch appearing in axial section inFIG. 2, taken along the line 4-4 in that figure;

FIG. 5 is a side elevation of the top punch appearing in FIGS. 2 and 4;

'FIG. 6 is a vertical section through the mold showing the powderedinsulating material partially molded, with punch parts and core pinsforming a cavity for later receiving powdered resistor mix material;

-FIG. 7 is a vertical section through the mold showing the partiallymolded insulating material after the core pins and punch parts have beenremoved;

FIG. 8 is a vertical section through the mold with parallel leads beingheld in place by a punch part and with powdered resistor material inplace in the cavity formed therefor and covered with additional powderedinsulating material;

FIG. 9 is a vertical section through the mold showing the fully moldedresistor in the die opening after the final molding;

FIG. 10 is a vertical section through the fully molded resistor; and

FIG. 11 is a fragmentary View showing a terminal lead in the form of awire with upset head extending into and closing off the bottom of acavity in a molded shell, which cavity is filled with a resistive powderto be molded into a resistive filament.

Referring to the drawings, FIG. 1 is a view of one form of resistor madeaccording to the present invention. The resistance element of theresistor is a U-shaped filament 20 embedded in an encapsulating mass 22of insulating material. Solid metal terminal blocks 24, integral with orconductively attached to parallel leads 26 are in intimate conductivecontact with the respective ends of the filament 20.

The filament 20 may be made of a mixture of suitable proportions of (1)material of high specific resistance, for example calcined carbon black,or graphite, or both, (2) filler material, for example silica powder,and (3) a binder, for example a thermosetting resin of thephenolformaldehyde type. These materials are thoroughly blended toconstitute the resistor mix, which will sometimes be referred to hereinas the core mix, inasmuch as it is placed in a cavity Within thesurrounding insulating material. By varying the proportion of the firstitem in the core mix the resistance of the finished resistor may begiven the desired value.

The encapsulating mass 22 may be made of a mixture of silica sand,asbestos, and binders such as thermosetting resins of thephenol-formaldehyde type. These materials are thoroughly blended toconstitute the shell mix.

The terminal connecting leads 26 may be made of copper wire, with an endportion forming a terminal head 24, preferably having a collar portion25 of approximately the same diameter as the filament 20, as shown inFIG. 11.

The apparatus for producing the resistor is illustrated more or lessdiagrammatically in FIGS. 2 through 11. A die block 30 has a die opening32 therethrough in which the resistor will be assembled and molded. Inthe drawings a single die opening is shown, but in commercial practicethe die block probably would be provided with a number of die openingsfor substantially simultaneous assembly and molding of resistors inquantity production. A forming part, herein called a shell pin, is shownat 34, and constitutes a cylindrical member with parallel plane andsurfaces perpendicular to the cylindrical axis of the pin. The shell pin34 fits into the die opening 32 with a close sliding fit and is providedwith the usual alignment plate 33 and back-up plate 35. The pin is madeof solid metal except that it contains two axially drilled holes 36extending through the part from end to end, as shown in end View in FIG.3. Another backup plate 38 is provided from which extend two core pins40 arranged to be inserted through the holes 36 in the shell pin 34, andthrough aligned holes in the back-up plate 35, as shown in FIG. 2, witha close sliding fit. Another alignment plate 37 is provided to fit overthe back-up plate 38 around the bases of the core pins 40 as shown. Atop punch 44 is provided which also makes a close sliding fit with thedie opening 32 and is bored with holes 46 which make a close sliding fitwith the core pins 40. The punch 44 has a bottom extension 48 and theholes 46 extend through the punch from end to end, including the bottomextension 48, as shown in FIGS. 2, 4 and 5.

With the shell pin 34 and core pins 40 in place in the die opening 32 asshown in FIG. 2, a preferably measured quantity of the shell mix isplaced in the die opening 32 in the space surrounding the core pins 40'.In accordance with conventional practice the die block will now berapped or vibrated to insure settlement of the shell mix in uniformlayers.

Next the top punch 44 is inserted in the die opening 32, sliding overthe core pins 40, and hydraulic pressure is applied between the baseplate 38 and the top punch 44, in known manner. FIG. 6 shows theconfiguration ob taining after the hydraulic pressure has been applied,upper and lower jaws of the hydraulic press being shown conventionallyat 50 and 52 respectively. In this operation, the shell mix is partiallycompacted and molded to such an extent that upon retraction of the pressjaws and removal of the top punch 44, the core pins 40 and the shell pin34, the shell mix will remain held by friction within the die opening 32and will retain its impressed shape due to compaction as shown in FIG.7. The molded shell mix now has the form of the die opening 32 as wellas two cylindrical cavities that have been formed by the core pins andan indentation at the top that has been formed by the bottom extension48 on the top punch 44 which joins the two cylindrical cavities todefine a U-shaped cavity.

Next, a molding punch 39 is provided of the same outside diameter as theshell pin 34 but with axial holes 62 of smaller diameter than the holes36 in shell pin 34. Leads 26, with enlarged or upset head 24 and collar25 are inserted into the holes 62 with the collars 25 resting upon theflat upper surface of the punch 39. The holes 62 are coaxial with thecavities 64 so that the collars 25 serve to close off the bottoms of thecavities 64 as shown in FIG. 8, when the punch 39 is slid up into thedie opening 32 until the punch presses against the bottom of the formedshell. To more effectively close off the cavity 64 and positively guidethe collar 25 to assume a coaxial relationship to the cavity 64, thecollar 25 is preferably provided with a short right cylindrical lowerportion 23 as shown in FIG. 11.

In an illustrative example, where the filament 20 of resistive materialhas a diameter of 0.093 inch, the terminal lead 26 may comprise a wireof 0.020 inch diameter with an upset head 24 that is about 0.030 inch indiameter and a collar 25 of 0.093 inch outside diameter, the collarhaving a flat lower surface to present a bearing surface to the topsurface of the punch 39. The holes 62 in the punch 39 may be of somewhatgreater diameter than the shank of the lead 26, for example, about 0.040inch diameter.

Then a suitable, preferably measured amount of core mix is poured intothe cylindrical cavities and into the depression formed by the extension48, settled in place and covered over with more of the shell mix, asalso shown in FIG. 8. In this figure, partically formed shell mix isshown at 54, settled core mix at 56 and the top covering of shell mix at58. The top covering 58 is preferably settled after the settling of thecore mix is completed.

A plain top punch 60, which may be a solid cylindrical bar with paralleltop and bottom surfaces perpendicular to its axis, is next inserted inthe die opening above the partially formed resistor and hydraulicpressureis again applied by jaws 50 and 52, this time the upper jaw 50bearing against the top of the top punch 60 and the lower jaw 52 bearingagainst the bottom of the molding punch 39 shown in FIG. 9. Thisoperation presses the resistor core mix in the cavities 64 firmly intointimate contact with the heads 24 and collars 25 and molds the resistorcore mix. It also completes the molding of the partially compacted shellmix 54 and molds the top covering of shell mix into an integral part ofthe whole. The pressure employed will be sufiicient to mold the resistorto the desired self-sustaining form and this may vary from a moderatepressure, for example about 1300 pounds per square inch, on up topressures much higher, depending on the materials used, the dimensionsof the resistors, and other factors. Following the molding operation theresistor will be ejected from the mold, for example by the top punch 60which may be made long enough to push the molded resistor out of the dieblock.

The molded resistor now has the form shown in axial section in FIG. 10and in perspective in FIG. 1.

After molding, the resistors may be cured in an oven to cause the resinto set. The curing temperature may vary depending on the particularthermosetting resin used, the time of curing, the size of the resistors,and other factors, but for phenol-formaldehyde resin may be somewhatless than 400 F. If desired, the curing may be done in a neutral(nitrogen) atmosphere, or in a reducing (hydrogen) atmosphere, althoughwith the preferred compositions described hereinabove this was not foundto be necessary to prevent oxidation of the metal terminals and leads.

Following the curing operation the resistors desirably will beimpregnated and coated with a wax or other resin to make them moreresistant to the effects of moisture. This treating material, may forexample, be of the type disclosed in the Veley Patent 2,313,853, Mar.16, 1943.

While I have shown and described apparatus for making a resistor inaccordance with the invention, it will be evident to those skilled inthe art that other circuit components involving combinations ofconductors and insulators, such for example as capacitors, may beproduced in similar manner by making suitable changes in the forms ofthe core pins and punch parts.

While illustrative forms of apparatus in accordance with the inventionhave been described and shown herein, it will be understood thatnumerous changes may be made without departing from the generalprinciples and scope of the invention.

I claim:

Apparatus for molding insulated resistors, comprising in combination, adie block having a die opening therethrough of predetermined shape toform the shape of the finished resistor, said die opening being ofsuitable shape to receive in close sliding fit a plurality of punchparts so that any said punch part can be slid through said die openingfrom end to end of the opening, first and second punch parts for saiddie opening, said punch parts having alignable openings therethrough toaccommodate in close sliding fit a pair of core pins, a base platesupporting a said pair of core pins in relative position to be slid intosaid alignable openngs when said punch parts are in place in said dieopening, a first of said punch parts having a projection on one endthereof, said projection extending from one said alignable opening tothe other so as to form a core spanning the space from one said core pinto the other when said first punch part receives said core pins withinthe said alignable openings in said punch part, means to compressmol-dable insulating material between said punch parts by pressureapplied between said base plate and said first punch part while saidpunch parts are spaced apart within said die opening and said core pinsare present in the said alignable openings, said base plate transmittingpressure against one end of the second of said punch parts; a thirdpunch part for said die opening, sai p ch part having a solid flat face,a fourth punch part for said die opening, said punch part havingopenings therethrough alignable with the said openings in said first andsecond punch parts and of slightly less diameter for holding terminalleads in place during molding and thereby preventing escape of moldableresistive material from the cavities formed by the said core pins, andmeans to apply pressure upon moldable material between said third andfourth punch parts after said base plate and core pins have been removedand the said cored cavity has been filled with moldable resistivematerial to complete the molding of the resistor.

References Cited by the Examiner UNITED J. SPENCER OVERHOLSER, PrimaryExaminer. J. HOWARD FLINT, 111., Examiner.

